This invention relates to thermionic cathode structures and, in particular, to an improved flat thermionic cathode formed on a substrate wherein the danger of damage to the substrate caused by thermal stress therein is substantially diminished.
Thermionic cathode structures are used in various vacuum and gas tube devices. Although many of such devices have been replaced by the advent of semiconductor technology, nevertheless, thermionic cathode structures are used as a source of electrons in cathode ray tubes (CRT), electron beam storage tubes and other electron beam devices. A typical thermionic cathode structure used in such devices, and particularly the CRT, may be assembled into an electron gun assembly formed of various control and accelerating grids whereby emitted electrons are shaped into a beam to scan a target. In general, such a thermionic cathode structure includes a metal tube or sleeve provided with a metal end wall. The outer surface of this end wall, that is, the surface facing away from the interior of the sleeve, is provided with thermionic electron emitting material, such as a coating of such material, whereby electrons readily are emitted therefrom when the coating is heated to a suitable temperature. The requisite heat is produced by a filament positioned within the metal sleeve, the filament being supplied with a heating current so as to maintain the proper temperature whereby electron emission occurs from the electron emissive coating.
This type of thermionic cathode structure, especially when provided in a color cathode ray tube used in color television receivers, is relatively difficult to assemble, thus requiring a highly skilled technician. Consequently, such a thermionic cathode structure has resulted in higher manufacturing costs and lower productivity in the manufacture of CRT's. For example, the metal sleeve of the cathode structure generally is supported by a ceramic disc which, in turn, is disposed within a cup-shaped control grid, the ceramic disc and cathode structure being particularly positioned within the grid such that the electron emissive coating is spaced from the end wall of the grid by a predetermined distance.
Accordingly, to avoid the problems of manufacturing and assembling such prior art thermionic cathode structures, and thus reducing the overall cost of manufacture, a flat thermionic cathode has been proposed. This proposed cathode structure is formed with an insulating substrate upon which a layer of resistive current conducting material is provided so as to form the heating element for the cathode. A portion of this heating element is coated with a layer of insulating material, and a layer of electron emissive material then is deposited upon at least a portion of the insulating layer. Hence, the metal sleeve and heating filament within the sleeve, heretofore typical of prior art thermionic cathode structures, are avoided.
Preferably, this flat cathode structure should be made as thin as possible. Accordingly, the substrate should be very thin so as to reduce the power consumption of the cathode heater element and, also, to reduce the time required for the electron emissive material to be sufficiently heated so as to emit electrons. Unfortunately, if the substrate is made thinner, there is a strong possibility that it may fracture or be otherwise damaged because of local thermal stress therein. That is, if the cathode heater element is provided in a relatively localized area so as to localize the heat applied to the electron emissive coating, a temperature gradient will be produced between the localized heating area in the substrate and, for example, peripheral areas of the substrate which are much cooler. This temperature gradient creates thermal stress in the substrate of a type which may cause fracturing, especially at the perimeter of the substrate.